The present invention relates to a glass composition, and more specifically to a glass composition capable of reducing bubbles in a glass product by decreasing a dissolved gas in a glass and capable of improving homogeneity of the glass product.
The most important object among objects addressed over many years in manufacture of a glass product is to manufacture a very homogeneous glass by completely removing bubbles remaining in the glass which lead to defects in the glass product. Various inventions have been made to attain the object, but a method is not yet discovered which satisfies the prerequisite of stable manufacture and supply of a high quality glass demanded by the industrial world and consumers at low cost and which is easy for glass manufacturers to implement.
Further, a method of manufacturing a glass product from a solid such as a method employing a gas phase reaction and a sol-gel method is recognized. However, in general, the most often employed industrial method of manufacturing a large amount of a glass product involves: using a mixture of inorganic compounds as a main raw material for glass raw materials; and melting the raw materials at high temperatures, to manufacture a glass product. According to the manufacturing method employing melting, methods of removing bubbles from a molten glass are roughly classified into a chemical method and a physical method. A typical example of the former method is a method of adding fining agents into glass raw materials, and a typical example of the latter method is a method of reducing pressure or vacuum degassing the molten glass.
In particular, while a wide variety of glass products are supplied recently, the former method generally involves: adding a minute amount of fining agents into the glass raw materials; heating the glass raw materials at high temperatures for foaming in a molten glass through their decomposition or redox reaction; and removing a carbon dioxide gas generated during melting, bubbles remaining in the raw materials, nitrogen generated during glass melting. The method is characterized in that, although attention must be focused on control of glass melting temperature and of molten glass flow and on segregation of fining agents in the glass raw materials, the method enables continuous mass production of a glass product with relative ease if a fining agent providing a stable fining effect can be selected. There exist many cases where such a fining method suitable for mass production of glass has been employed during melting.
Among those, the most frequently employed method involves adding arsenic, which provides a sufficient fining effect with minute addition, as oxide in the glass. However, environmental problems have been pointed out regarding addition of arsenic. Thus, an urgent need of selecting another fining agent for replacing arsenic or for reducing the amount of arsenic added as a fining agent has resulted in reexamination of other hitherto proposed fining agents, developments of new fining compounds, or the like. Among those, many substances such as antimony, chlorine, and niter have been studied as substitute for arsenic, but a fining agent having a fining effect surpassing that of arsenic is far from found, particularly for an oxide glass requiring high temperature melting. Thus, melting furnace conditions have been studied for aiding the fining effect of the fining agent replacing arsenic, and many attempts have been made on various measures for solving the problems such as combination of a plurality of fining agents and increase of furnace temperature. However, despite such continuous studies and attempts, a low-cost method applicable to various glass compositions and assuredly providing a stable and good fining effect is not yet found.
On the other hand, examples of the physical method include: reduction of glass viscosity by increasing a melting temperature; a centrifugal method; flow control of molten glass inside the furnace; a stirring method; a gas blowing method; a sonic/ultrasonic method; an reduced pressure method; control of a melting atmosphere; and a combination thereof. Several inventions have been reported regarding a method of forcibly accelerating a rise of bubbles in the glass to the surface by keeping the molten glass under reduced pressure, for example. Patent Document 1 and Patent Document 2 below each describe a reduced pressure defoaming apparatus arranged between a melting tank and a working tank of a glass melting furnace. However, such methods are not yet widely used as a general method of homogenizing a glass because the methods are not as easy as the chemical method to implement, do not provide comparable results as those of the chemical method, require a very large capital investment, and limit a usable glass composition.
Various gases are employed for a melting atmosphere in a part of a glass manufacturing process. Among those, Patent Document 3 below describes a method involving remelting a glass in an inert gas atmosphere as a means for preventing reboiling. Further, Patent Document 4 below describes use of a hydrogen gas or a helium gas during a densing process of a quarts glass tube base material. Further, Patent Document 5 below describes reduction of a water content in a glass by bubbling a gas selected from the group consisting of C02, N2, O2, NOx, and a noble gas as a dry gas.
Further, Patent Document 6below describes that a supplemental effect can be realized during fining of a molten glass by using helium with sodium chloride, and that the effect was confirmed upon using helium with a very small, experimental amount of the molten glass.
Patent Document 1: JP 11-130442 A (p. 2-7, FIGS. 1-2)
Patent Document 2: JP 11-130444 A (P. 2-7, FIG. 1)
Patent Document 3: JP 06-329422 A (p. 2-4)
Patent Document 4: JP 09-301726 A (p. 2-4, FIG. 1)
Patent Document 5: JP 07-172862 A (p. 2-8, FIGS. 1-7)
Patent Document 6: U.S. Pat. No. 3,622,296
Defoaming of a molten glass under reduced pressure as in Patent Document 1 and Patent Document 2 may be recognized as one measure, but such a method requires large-scale remodeling of a manufacturing facility and may require introduction of an expensive devices, thereby being disadvantageous to attain a reduced cost price for manufacture of a glass product requiring mass production.
Further, adoption of the inert gas for the melting atmosphere as in Patent Document 3 and Patent Document 4 has been realized for specific glass compositions. However, the method is for shielding the molten glass from oxygen and for adjusting the water content. Neither Patent Document 3 nor Patent Document 4 describes an inert gas content in the glass or a fining action during glass melting.
On the other hand, a fining method in Patent Document 6 had focused on the helium gas for the first time, and was an innovative fining method for the molten glass. However, the method was regarded supplemental to specific glass compositions, and assumes nothing on quantitative criteria of helium usage for broader applications, applications to glass materials of other grades, or the like. Thus, no follower of the method appeared, resulting in no application of the method to a widely used multicomponent oxide glass and no new improvements.